Fix loading of OCS routes for apps

[nickvergessen]

Regression from: #18109

@DeepDiver1975 @Xenopathic @LukasReschke

This broke all OCS routes of apps => no smashbox possible anymore.

[scrutinizer-notifier]

A new inspection was created.

[LukasReschke]

I'm wondering whether we should not just revert #17961 and move the required logic to lib/private/app.phpin L434. I really can't judge if this will fix all edge-cases or if there still will be things that break.

[LukasReschke]

Basically do a check for the configured theme URL in app.php (just check if the configured value is empty) and if not do the linking there just as before. Then the theme class does not require the URLGenerator.

[RobinMcCorkell]

I would suggest not even loading any OCS routes if there are unavailable apps. With this approach funky things might happen when OCS routes are reloaded once apps become available, especially with $this->root->addCollection() being called multiple times with duplicated content. At best, this can become a performance issue, at worst, the duplicate routes confuse something else to the point of breakage.

[ghost]

🚀 Test PASSed.🚀

[DeepDiver1975]

let's revert this mess and retry from scratch

[DeepDiver1975]

@nickvergessen will open a revert pr - THX